In aviation applications, fuel delivery systems for gas turbine engines typically utilize high pressure, positive displacement pumps to supply high pressure fuel to the engines which power the aircraft. In addition, the high pressure fuel system is often utilized as a source of high pressure fluid for the hydraulic systems and servos which position actuators that control the engine or other aspects of the aircraft.
The fuel pump is typically driven by the turbine engine through a gearbox. The pump flow rate is thus proportional to engine speed. The main fuel supply pump is sized to supply enough fuel to the engine during windmill start conditions, which are typically about 6 to 10% above normal cruising speed, and/or during maximum power conditions. Accordingly, at many engine operating conditions, the engine flow demand is significantly less than the high pressure flow supplied by the main pump. The excess high pressure pump flow is typically bypassed back to the low pressure inlet of the pump. Raising the pressure of the excess flow and returning the excess flow back to low pressure is effectively wasted energy. This energy is realized as heat input into the fuel and results in undesirable higher fuel temperatures and also requires the addition of large heat exchangers for removing the excess heat from the fuel
There have now been attempts to generate fuel delivery systems that include two separate pumps for pumping fuel. One pump, often referred to as a main pump or a main metering pump, provides fuel to the engine during normal operating conditions. The other pump, often referred to as an actuation pump, supplies fluid to various actuators throughout the aircraft.
To avoid over sizing the main pump for the majority of its operating conditions, the main pump may be sized to supply fuel at a rate less than which may be required during relight operations or other high flow demand situations. Thus, some fuel delivery systems have been implemented that provide flow sharing that couple the actuation pump to the main pump such that the actuation pump can supplement the fuel supplied to the turbine engine during these high fuel demand situations.
In these situations, the discharge pressure of the actuation pump must be at or above the discharge pressure of the main pump to avoid the effects of system transients. Thus, the discharge pressure of the main pump can be used to regulate the discharge pressure of the actuation pump. However, the applicants have determined that it can be problematic to use the discharge pressure of the main pump for regulating the discharge pressure of the actuation pump at all times. More particularly, to accurately control the discharge pressure of the actuation pump using the discharge pressure of the main pump requires that the parameters needed to determine the main pump discharge pressure be known. However, these parameters may include engine speed, CDP, and flow split. Further, as the discharge pressures of the actuation pump must be increased near the main pump must discharge pressure to minimize switching transient. This requires that the parameters need to anticipate when the flow sharing will occur.
There is therefore a need for a fuel supply system where the discharge pressure of the actuation pump is independent of external parameters during normal operation, but can be overridden when entering a flow sharing mode.